Hi Jellyworm, I am not too far away from you, just the other side of the Humber bridge. I had the 3.8 but sold it as it was too heavy for me on my own but did look into the transducer mount.
I think the arm needs to be as short as possible to reduce vibration or to use a heavier gauge of material to aid this issue. The wooden bracket idea is a good base to build on although I would be tempted to use a length of aluminium angle mounted on the transom with a heavy weight nylon cutting board, 12mm thickness shaped out to offer strength but at the same time very little resistance in the water.
Hmmm, not so easy to explain this and no photo to add. Imagine something like a rudder with the transducer fastened to the bottom of the blade.
Hope that helps, not so sure it does.